Improved XLFormOptionsViewController customization support

[tarbayev]

I needed to display images in options cells. So I extended XLFormOptionObject with an optional method configureCell:. By implementing this method objects can perform extra customization on a cell provided.

I also added cellReuseIdentifier optional property, which allows the option to be displayed by a custom cell.

And one small sort of fix: if tableView property has been set for XLFormViewController there should be no extra manipulation with the view hierarchy. This allows us to set tableView as a view controller's root view.

[WA-Will]

Can I ask how exactly you did this? I've been trying to change the background color of cells for a few days now and can't quite get it.

[tarbayev]

Did what? Normally you change the background color of a cell by setting view with the color you need to its backgroundView property. Unfortunately XLForm is not very extensible and it's impossible to access a cell directly to make additional setup. You can only set some attributes in configuration dictionaries of XLFormRowDescriptor which are set to cell with setValue:forKey: method.
What I did, I've implemented an NSObject category with a block property. In a setter block is called with self as its argument (https://gist.github.com/tarbayev/c1cccbb6d2fdfa1227cb). So it's basically a visitor pattern implemented with block. Setting this property name as a key to configuration dictionary of XLFormRowDescriptor I'm able to access its cell and do any additional setup.